The present invention relates to a semiconductor device and a method of fabricating the semiconductor device, and particularly to a semiconductor device in which solder bumps are formed on electrode pads via a barrier metal film is and a method of fabricating the semiconductor device.
To increasingly progress miniaturization of electronic equipment, it is important how improve the mounting density of electronic parts. With respect to semiconductor ICs (Integrated Circuits), studies have been actively made to develop such a high density mounting technology as to directly mount a bare chip on a printed wiring board, typically a flip-chip mounting method in place of the conventional packaging method.
The flip-chip mounting method is represented by an Au (gold) stud bump method or a solder ball bump method, and further, it includes various other methods covering a wide range from those at the prototype stage to those at the commercially practical stage. The present applicant has developed in advance of time a mass-production technology for forming solder ball bumps on a wafer level including a number of chips by applying the LSI (Large-Scale Integrated Circuit) fabrication process, and has filed a number of the associated patent applications and simultaneously put consumer-oriented equipment produced on the basis of such a technology, for example, a micro digital camcorder, to thus early commercialize the next-generation LSI mounting technology.
In formation of solder ball bumps, a barrier metal film is provided between Al (aluminum) based electrode pads of an LSI and the bumps in order to improve the adhesion between the electrode pads and the bumps and prevent the mutual diffusion therebetween. In particular, for the solder ball bump method, since the barrier metal film exerts a large effect on the finished shapes of bumps, such a film is generally called a BLM (Ball Limiting Metal) film.
In general, the BLM film used for solder bumps has a three-layer structure of Cr (chromium) film/Cu (copper) film/Au film. In this three-layer structure, the bottom Cr film functions as an adhesion layer for ensuring good adhesion with Al electrode pads; the intermediate Cu film functions as a barrier layer for preventing the diffusion of solder from solder bumps; and the top Au film functions as an oxidation preventive film for preventing the oxidation of the Cu film.
A related art method of forming solder bumps using such a BLM film will be described below with reference to FIGS. 8 to 12.
First, an Al based electrode pad 32 made from Al or an Al—Cu alloy is formed on a connection of a flip-chip IC (not shown) formed on the surface of a semiconductor substrate 30. The entire surface of the substrate is covered with a passivation film (surface protective film) 34 made from polyimide film or silicon nitride film, and a BLM film 36 is formed in such a manner as to be connected to the Al based electrode pad 32 via a connection hole opened in the passivation film 34 (see FIG. 8).
The entire surface of the substrate is coated with a sufficiently thick photoresist film 38, and the photoresist film 38 is patterned by photolithography to form an opening 40 having a diameter being large enough to expose the BLM film 36 and a portion of the passivation film 34 around the BLM film 36 (see FIG. 9).
A solder vapor-deposition film 42 made from Pb (lead) and Sn (tin) is formed over the entire surface of the substrate by use of, for example a vapor-deposition method. At this time, since the end, positioned at the edge of the opening 40, of the photoresist film 38 is largely stepped, the solder vapor-deposition film 42 is divided into a solder vapor-deposition film 42a located on both the BLM film 36 and the portion of the passivation film 34 around the BLM film 34 in the opening 40 and a solder vapor-deposition film 42b located on the photoresist film 38 (see FIG. 10).
The solder vapor-deposition film 42b located on the photoresist film 38 is removed together with the photoresist film 38 by a lift-off method. The lift-off method is performed by dipping the wafer in a resist separation solution and heating/oscillating the wafer in the solution, to lift-off the photoresist film 38. In this way, only the solder vapor-deposition film 42a, which covers the BLM film 36 and the portion of the passivation film 34 around the BLM film 36, remains (see FIG. 11).
The solder vapor-deposition film 42a is then subjected to wet-back treatment. That is to say, the solder vapor-deposition film 42a is coated with flux and is fused by heat-treatment, to finally form a solder ball bump 44 connected to the BLM film 36. In this way, a semiconductor device is fabricated, in which the solder ball bumps 44 are formed via the BLM film 36 on the Al based electrode pads 32 formed on the connections of the flip-chip IC formed on the surface of the semiconductor substrate 30 (see FIG. 12).
While Al has been used as an interconnection material of LSIs for a long time, the use of Al has come to cause serious problems associated with delay of signals or deterioration in the reliability in multiple interconnection layers along with tendency toward higher integration, finer-geometries, and higher operational speed of LSIs.
In place of such a conventional material Al, Cu (copper) is expected as an interconnection material of the next-generation LSIs and is nearing practical use. Cu has a resistivity lower about 40% than that of Al (the resistivity of Al is about 2.8 μΩcm while the resistivity of Cu is about 1.7 μΩcm), and further Cu has a high resistance against electromigration. Accordingly, the use of Cu is expected to further reduce the resistance of the LSIs and improve the reliability thereof.
However, since the above-described process of forming solder ball bumps on electrode pads via a BLM film is predicated on the use of Al based electrode pads, if such a process is applied to an LSI using Cu electrode pads as terminals of Cu interconnections in place of the conventional Al based electrode pads as terminals of Al based interconnections, there may occur the following problem. Namely, since the adhesion strength between the BLM film and the Cu electrode pads is lowered, there may easily arise the falling of solder ball bumps from a semiconductor chip upon mounting of the semiconductor chip on a printed wiring board or the failure in electric contact characteristic between the Cu electrode pads and the solder ball bumps when the semiconductor chip undergoes a temperature cycle or a high temperature load, thereby exerting adverse effect on the device reliability.
Consequently, it is desired to establish a fabrication process of stably forming solder ball bumps with a high-reliability to the next-generation LSIs adopting a Cu interconnection material.